When producing laminated beams melamine-based amino resins, usually melamine-formaldehyde- (MF) and melamine-urea-formaldehyde (MUF) resins, are commonly used. These amino resins give light-coloured joints in contrast to phenolic-based resins such as phenol-formaldehyde- or phenol-resorcinol-formaldehyde resins. UF resins are not suitable for use when making laminated beams due to their low water resistance. As curing agent for an MF or MUF resin an acid is used. The most commonly used acid when producing laminated beams is formic acid, or another volatile acid, for example acetic acid. Formic acid gives good bonding results but negative side-effects to the working environment and increasing corrosion on the gluing equipment. Non-volatile acids which give sufficient curing rate (e.g. para-toluenesulfonic acid (PTSA) and maleic acid) are quite strong acids which increases the risk of acid damage to the wood surface and gives poor stability of hardener compositions also containing polymer dispersions.
When gluing laminated beams at low temperatures and low humidity in the air, there has been seen a problem with sufficient bond quality when using state of the art adhesive systems using formic acid as the curing agent.
It would be desirable to provide a hardener composition which does not create the problems to the working environment as volatile acids do. Furthermore, it would be desirable to provide a hardener composition, and an adhesive system, which give improved results in terms of gluing quality, when used in low temperature, low humidity environments.
WO 01/70898 A1 discloses an adhesive system comprising an etherified amino resin, a carboxylic acid, polyvinyl acetate and polyvinyl alcohol.
U.S. Pat. No. 6,436,865 discloses a liquid catalyst for crosslinking with amino resins.
WO 2005/010119 discloses an adhesive composition comprising an amino resin and an acid with a pKa lower than 6.
It is an object of the present invention to provide a hardener composition which in combination with an amino resin, in particular melamine-formaldehyde- or melamine-urea formaldehyde resins, provides an adhesive system which when used in wood gluing methods, preferably when producing laminated beams, gives a strong adhesive bond, e.g., measured as fiber tear and delamination, and no or low acid damage on wood surfaces, and possibility of long assembly time. The hardener composition should furthermore not create the problems with corrosion and the problems to the working environment, as volatile acids do. The hardener composition should also preferably be storage stable.